Historically, there was no Eagle taken at the battle of Talavera, though it was a victory for the British.
The South Essex Regiment is entirely fictional and has never existed within the British Army.
The book Sharpe's Eagle was the first published Sharpe novel, although chronologically it is eighth in the series.
Major Lennox mentions that both he and Sharpe participated in the 1803 battle of Assaye. In the book series, it was at this battle where Sharpe saved the life of the future Duke of Wellington and was made an officer.
This is the first of the Sharpe movies to introduce a running theme of cultural references, here regarding art (Peter Paul Rubens and Sandro Boticelli) and, as a recurring theme for the future, literature in general and Shakespeare in particular (here: "Julius Caesar").